Zener diodes are specially designed diodes (heavily doped) that have low reverse voltage breakdown.
Due to this characteristic zener diodes are connected backwards to normal operation. If zener is forward biased it acts as regular diode with forward voltage drop at 0.6V. Zener diod backward voltage breakdowns may range from 2.4V up to 100V. Truly speaking if you need like 1.2V then probably you need to connect two forward biased diodes in series for 0.6V+0.6V = 1.2V drop.
In general purpose diodes we want reverse breakdown voltage to be as high as possible, in zener diodes we want this voltage to be what we need. For instance we may need diode breakdown voltage to be 12V. We can see from characteristic above, that once diode voltage is broke at some point it stays pretty stable even if reverse current varies largely. This is why zener diodes are commonly used as voltage regulators. See at standard regulator circuit.
We can see that circuit is very inefficient especially when higher current is needed on output. This is because current has to flow through diode protection resistor R. Lets do some interesting calculations here to see whats happening. First of all lets select real zener diode. For this purpose lets chose 1N4728A 3.3V 1W zener diode. From datasheet we can find that maximum current in diode is 76mA. So we have to make sure that this current won’t go higher. For this current limiting resistor R is used. Lets calculate resistor value. Lets say input unregulated voltage is 5V then resistor value:
R=(Vin-Vzener)/Izener = (5-3.3)/76mA = 22.7Ω.
As we don’t want to operate at the limits lets choose resistor to be standard 27Ω. This means when no load is applied to output all current floating through resistor will flow through zener as well. With R=27Ω we get current 63mA. And thus already we dissipate P = 0.063 * 27 = 1.7W of power on resistor. For battery operated devices this is a lot. But wait if we attach load to output. Say we want to power microcontroller device with requires 100mA. Then all this current has to flow through resistor as well. So power dissipation then P = (0.1 + 0.063) * 27 = 4.4W and this goes up when more current is needed by load. The more current the more resistor turns in to heater. As you can see resistor power rating has to be selected carefully if you don’t want to see smoke going up. This is why it is better to use specialized voltage regulators in such case.
But don’t worry zener diodes have more uses that are even more important than just voltage regulation. Zeners are also used as amplitude limiters or so called clippers. They are great to protect input circuits from voltage spikes in signals that could damage further circuit:
When signal amplitude is lower than zener breakdown voltage diodes have almost no effect on signal, but when signal exceeds zener voltage breakdown voltage it clips signal to zener voltage. Also this is one of ways to generate near square signal out of sine.